Public health officials are nearly certain that the Zika virus is behind Brazil’s surge of babies being born with tiny heads and damaged brains, but they do not expect to have proof until June at the earliest, the World Health Organization said on Friday.
“It will probably be four, five, six months,” Dr. Bruce Aylward, the organization’s head of emergency response, said at a news conference in Geneva. But he added that the evidence was already so strong that “at this time, the virus is considered guilty until proven innocent.”
Experts, he said, are waiting for a cohort of about 5,000 pregnant women, most of them in Colombia, to give birth.
Those women, all of whom had Zika infections early in their pregnancies, were enrolled in a large clinical trial that effectively started in October, so most are expected to start giving birth in June. They will be tested to be sure they do not have any other known causes of microcephaly — a birth defect characterized by small misshapen heads — such as rubella, toxoplasmosis or genetic mutations. Researchers will compare their babies to those of a large group of pregnant women who did not have Zika.
If the infected group has significantly more microcephalic babies, researchers can say with confidence that the virus was the cause.
When the World Health Organization declared an international public health emergency on Feb. 1, it said it was doing so to ascertain whether Zika caused microcephaly. The virus alone was not grounds for an emergency declaration, the health agency said, because it causes relatively mild disease in most people.
Dr. David L. Heymann, a professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine and the chairman of the W.H.O.’s advisory panel that recommended declaring the emergency, said then that such a comparison of two groups equal in all ways except for one important factor — known as a case-control study — was needed to prove causation.
In October, some doctors in Latin American countries began tracking women who had Zika while pregnant. They did so because hospitals in northeast Brazil had just reported a huge surge in microcephalic births. The surge began in August, and, at its peak, the region had 20 times its normal number of such births.
The Brazilian doctors noted that, about nine months earlier, the northeast had experienced an epidemic of a “doença misteriosa,” or “mystery disease,” that caused a rash, fever and bloodshot eyes. It was later identified as Zika, which is transmitted by mosquitoes.
That same October, Colombia was being hit hard by Zika for the first time, with so many newly infected pregnant women that it created an opportunity to study them.